They're special bolts that engage in the slots in the Multipla roof mountings. Instead of having a round or hex head like a screw or bolt, they have a short piece of 'bar' about the same diameter as the bolt shank but at right angles to it, as a head.
If you haven't got the T bolts, there's no way to fix the roof bars on the car.
In answer to the original question, I think you'll struggle to get replacements on their own. If you had one to copy, you may be able to weld something up from a standard bolt(s) that would do the job. When I get time, I can dig mine out of the garage and measure them up if that would help.